Electro-optic storage device



May 20, 1969 R. A. MYERS ELECTRO-OPTIC STORAGE DEVICE Sheet Filed Jan.5. 1966 y M m w L 2. on Eas INVENTOR. ROBERT A. MYERS Ll /MJJ ATTO R NEY May 20, 1969 R. A. MYERS ELECTRO'OPTIC STORAGE DEVICE Sheet FiledJan. 3, 1966 HE :8 M52 5:532 mwm tinited rates Patent: fiice 3,445,826Patented May 20, 1969 3,445,826 ELECTRO-OPTIC STORAGE DEVICE Robert A.Myers, i *mvYork, N.Y., assignor to International Business MachinesCorporation, Arruonk, N.Y., a corporation of New York Filed Jan. 3,1966, Ser. No. 518,369 Int. Cl. Gllb 7/00; H015 3/00 US. Cl. 340-473 10Claims This invention relates to optical apparatus, and moreparticularly to devices which exhibit elcctro-optic effects in responseto charge patterns deposited thereon.

The electro-optic effect has been employed to convert an electric fieldpattern into a pattern of light. Often the material exhibiting theelectro-optic effect is illuminated with polarized light, and the planeof polarization is altered in accordance with the field pattern. Thefield may be created for example, in a cathode ray tube wherein a beamof electrons is directed against a face composed of electro-opticmaterial. While the electron beam can be moved very rapidly across theface of the tube, the charge pattern deposited thereon cannot be rapidlyerased to permit a new pattern to be displayed.

Flood guns may be employed in the cathode ray tube to erase the chargepattern from the face by heavy bombardment of electrons of the properenergy and consequent secondary emission from the face. However, suchfiood guns require the presence of an additional grid near the face ofthe tube to carry off the secondary emission charge, interfering withthe primary electron beam which must pass through the grid. Also, theflood gun adds a considerable amount of hardware to a cathode ray tube.

Accordingly, it is an object of the present invention to provide anelectro-optic device capable of rapid changes.

Another object of the present invention is to provide an improvedelectro-optic device capable of rapidly erasing charge patternsdeposited thereon.

Still another object of the present invention is to provide a cathoderay tube having an electro-optic face and Capable of rapid erasure ofthe electron beam charge deposited thereon.

A further object of the present invention is to provide an improvedelectro-optic device having the ability to erase charge patterns storedthereon without interfering with the resolution of the charge patterndeposited thereon.

It is another object of the present invention to provide anelectro-optic device capable of storing a charge pattern thereon for avariable period of time.

Another object of the present invention is to provide apparatus for usein laser mode selection.

These and other objects of the present invention are accomplished byproviding an electro-optic device which also exhibits a photoconductiveeffect. The charge pattern deposited on the device is erased byillumination.

In accordance with one aspect of the present invention, a source ofpolarized light is directed onto the device exhibiting a photoconductiveand electro-optic effect. The frequency range of the polarized light isselected to be outside the range of frequencies capable of producing aphotoconductive effect in the device. On the other hand, the source ofillumination used to erase the charge pattern is selected to be withinthe range of frequencies causing the device to be conductive.

In this manner, there is no interference with the deposition of chargeon the electro-optic device. If an electron beam is used to deposit thecharge pattern, the source of illumination for erasure purposes may belocated out of the way of the electron beam. Further, since erasure isaccomplished using a photoconductive effect instead of asecondary-emission effect, the deposited charge may be carried offwithout the use of an additional grid interfering with the resolution ofthe device.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more" particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

In the drawings:

FIG. 1 is a diagram illustrating an electro-optic device employing thepresent invention to control the direction of emission of laser light;and I FIG. 2 is a diagram illustrating another embodiment of the presentin ention employed in a display system.

In the apparatus of FIG. 1, the direction of emission of laser lightfrom a ruby 10 is controlled by a cathode ray tube 12. An electron beam14 selects one of a plurality of angular modes of oscillationrepresented by a mode 16 in FIG. 1.

Laser emission occurs along the mode of oscillation 16 due to theelectro-optic effect in a crystal 18. At the location where electronbeam 14 strikes the crystal 18, a charge is deposited setting up a fieldacross crystal 18 which alters the refractive indices of the crystal 18at this point.

A lamp 20 is employed to discharge the field set up across crystal 18.The material of crystal 18 is selected to exhibit a photoconductiveeifect in response to the illumination of lamp 20. When lamp 20 isactivated, the charge deposited by beam 14 is drained off erasing thefield and thereby extinguishing the preferred mode of oscillation 16.

Detailed description of embodiment in FIG. 1

Cathode ray tube 12 operates in a conventional manner. A battery 22supplies the acceleration voltage for beam 14.

- On the outer surface of crystal 18, a transparent coating 24 ofstannous oxide (SnO) provides a ground reference for electron beam 14. Adielectric mirror coating 26 is applied to the inner surface of thecrystal 18 forming one end of a laser cavity. The other end of the lasercavity is formed by a mirror 28.

A pair of lenses 30A and 30B focus the angularly oriented mode ofoscillation 16 on the reflecting surfaces 28 and 26 respectively. Othermodes of oscillation (not shown) similar to modes 16, but rotated aboutthe center of laser 10 are also focused on the reflecting surfaces 28and 26 by lenses 30A and 30B respectively. One specification for lenses30A and 30B may be found in commonly assigned co-pending applicationSer. No. 511,775, filed Dec. 6, 1965 entitled, Lens Group Employed inLaser Cavities, by R. E. Tibbetts and J. S. Wilczynski.

A polarizer 32 is placed in the path of all modes of oscillation totransmit light only with a single axis of polarization. A fixed opticalretardation plate 34 introduces a relative phase delay betweenorthogonal polarization components of light passing therethrough. Theplate 34 is arranged so that the two orthogonal components are orientedat 45 degrees with respect to the polarizer 32. In a like manner, thecrystal 18 is oriented so that its electrically induced principal axesare oriented at 45 degrees to the polarizer 32. Reference may be made tocommonly assigned co-pending application Ser. No. 412,814, entitledApparatus for Controlling a laser Beam by R. V. Pole and R. A. Myers,for further details of the polarizer 32, and plate 34, as Well as for amore detailed description of the operation of the laser cav ty than ageneral description to follow.

In operation, a pumping source 36, typically a helical fiash tube,stimulates laser 10 so that light is produced along a large number ofangularly oriented modes of oscillation, such as mode 16. The lightpassing through polarizer 32 is limited to a single polarization axis.After passing through polarizer 32 and retardation plate 34, a

phase delay exists between two orthogonal components of the light. Afterreflection from mirror 26, the light undergoes an additional phase delaypassing through plate 34 and a portion thereof is blocked by polarizer32, which acts as an analyzer for this light returning to laser 10.

The phase delay introduced by plate 34 is selected so that the intensityof the light returning to laser is insufficient to cause the laser tooscillate and emit its characteristic light in response to pumpingsource 36. In order to obtain laser emission, the phase delay introducedby plate 26 must be compensated for by an opposite phase delayintroduced by crystal 18. This is ac complished by the deposition of acharge on crystal 18 producing a field across crystal 18. This field inturn produces an electro-optic effect in crystal 18 altering thepolarization of the light passing therethrough to compensate for theretardation introduced by plate 34. Therefore, with the electron beam 14in the position shown in FIG. 1, laser 10 emission occurs along the mode16 passing out of the cavity through partially transmitting reflector28.

Laser emission along other modes of oscillation may be achieved bymoving the position of electron beam 14. However, the charge placed uponcrystal 18 must be removed it emission along mode 16 is no longerdesired. Crystal 18 stores the charge placed thereon for a period oftime determined by its characteristic ability to conduct the chargethrough to the ground potential coating 24. Crystal 18 may be composedof several difi'erent types of materials, such as zinc sulphide (ZnS),gallium arsenide (GaAs), gallium phosphide (GaP), silicon carbide (SiC),and zinc telluride (ZnTe). Crystalsof these materials, and others, maybe doped with impurities to alter their conductive property, and arereferred to as semi-conductors. Therefore, the storage time of thecharge placed on the crystal 18 by electron beam 14 may be varied bydoping the crystal in a manner well known in the semi-conductor art.This storage time is characteristic of the crystal 18 and permanentlyinstalled during manuacture thereof.

In order to change the storage time of the crystal 18, lamp 20 isprovided. The illumination from lamp 20 is selected to be in the rangeof frequencies producing a photoconductive effect in crystal 18. Each ofthe materials listed above exhibits a photoconductive effect for certainfrequencies of light. This effect may be enhanced by adding impuritiesto the crystal 18 during manufacture in accordance with well knowntechniques in the art. Assuming crystal 18 is made of a zinc sulphide(ZnS) material, lamp 20 provides illumination in the ultraviolet region.Therefore, the light from the ruby laser 10, typically in the redfrequency range, does not produce a photoconductive effect in thecrystal 18, while the ultraviolet illumination from lamp 20 causes alarge change in the conductivizy of crystal 18. The ratio of the lightand dark conductivity may be made up to 10 :1.

When illuminated the crystal 18 conducts the charge deposited on theside by beam 14 through to the grounded coating 24. Alternatively, abattery 40 connected by an electrode 41 to the upper portion of crystal1S, and another electrode 43 connected to ground may be employed todrain off the charge when crystal 18 becomes conductive. By arrangingthe electrodes 41 and 43 as shown in FIG 1, the field produced therebyis transverse to the field produced by electron beam 14 and may beoriented so it does not alter the operation of the polarized laserlight.

The lamp 20 may be operated in a variety of manners. One example isshown in FIG. 1, where a pulse source 42 is connected across the lamp 20to provide a short burst of illumination. The pulse source 42 may beused where an entire pattern is drawn on the crystal 18 by moving thebeam 14. Prior to drawing another complete pattern pulse source 42provides a burst of illumination from lamp 20 which erasesthe entirepattern. This operation is similar to successive frames in a motionpicture system.

Still another method of operation can be performed by a battery 44 andvariable resistor 46. in this case, the lamp 20 provides a constantillumination increasing the inherent conductivity built into the crystal18 during manufacture. Therefore, if it is desired to increase thestorage time of crystal 18, then variable resistor 46 may be increasedresulting in a decreased illumination, causing a corresponding increasein the storage time of crystal 18. Therefore, in the system of FIG. 2,the storage time may be varied by adjusting the constant illumination oflamp 20, and additionally by activating pulse source 42 for completeerasure.

Detailed description of embodiment in FIG. 2

Like numerical designations are applied to the same elements in bothFIGS. 1 and 2. One modification is the addition of a layer 1813exhibiting photoconductive effects to the back of mirror 26. A crystal18A in the same position as crystal 18 exhibits only the electro-opticetfect. This permits the selection of different materials for crystal18A and layer 18B. For example, crystal 18A may now be composed ofpotassium dihydrogen phosphate (KDP) which does not exhibit aphotoconductive effect.

Another difference in FIG. 2 is the source of polarized light whichincludes a lamp 50, passing light through an aperture 52. A lens 54collimates the light into parallel rays represented by a group of rays56 in FIG. 2. A filter 58 and polarizer 32A pass light of a singlefrequency and single polarization. The light rays 56 continue through abeam splitter 60, through the crystal and are reflected back off thebeam splitter. Before being viewed, the light passes through a secondpzfarizer 32B, crossed with respect to polarizer 32A. Thereic-re, in theabsence of a field pattern across the electro-optic crystal 18A, theviewer sees no light. The light pattern reaching the viewer correspondsto the charge pattern deposited by electron beam 14.

Lamp 20 operates in the same manner as described with regard to FIG. 1.Layer 18B becomes conductive when lamp 20 is illuminated by pulse source42 thereby draining off the charge pattern.

The embodiments of FIGS. 1 and 2 illustrate two of the many possiblelight sources, laser 10 and lamp 50, which may be employed in thepresent invention. Further, the photoconductive effect and theelectro-optic effect can be contained in a single crystal 18, or can beememployed in the present invention using two materials 18A and B.

Still another modification can be made to the embodiment of FIG. 2 wherea type of cathode ray tube is employed having the source for electronbeam 14 located off center and out of the path of rays 56. For thismodification, the observer views the light pattern from behind thecathode ray tube 12 and the necessity of a beam splitter 60 and mirror26 is removed.

While both embodiments shown in FIGS. 1 and 2 cmploy a cathode ray tube12 to deposit a charge pattern, many other devices may be employed toset up a field across the crystals 18 such as a metallic matrix on theback of the crystal 18, connected to a control source.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. An electro-optic device comprising:

storage means exhibiting electro-optic and photoconductive effects;

charging means for depositing a charge pattern on said storage means toalter the refractive index of portions thereof; and

control means for. illuminating said storage means to alter theconductivity thereof and to control the discharge of said depositedpattern.

2. Apparatus as defined in claim 1 wherein said charging means includesmeans for directing an electron beam against said storage means.

3. Apparatus as defined in claim 1 wherein said control means includes alight source of variable intensity.

4. Apparatus as defined in claim 3 wherein said charging means includesmeans for directing an electron beam against said storage means.

5. Apparatus as defined in claim 1 further characterized by the additionof a source of polarized light directed at said storage means andcomposed of a range of frequencies having substantially nophotoconductive effect on said storage means whereby the electro-opticeffect of said storage means alters the polarization of said polarizedlight in accordance with said charge pattern.

6. Apparatus as defined in claim 5 wherein said source includes a lasercavity having a plurality of modes of oscillation, and at least onereflecting surface located behind said storage means to cause said modesof oscillation to pass through said storage means, whereby the mode oflaser emission is selected by the position of the deposited charge.

7. An electro-optic device comprising:

a cathode ray tube having a face composed of a material exhibiting anelectro-optie and photoconductive effect; and

control means for illuminating said face to alter the conductivitythereof.

8. Apparatus as defined in claim 7 wherein said control means includes asource of light of variable intensity having frequency components withina range of those capable of producing a photoconductive etiect in saidcathode ray tube face.

9. Apparatus as defined in claim 8 further characterized by the additionof a source of polarized light directed at said face and composed offrequencies outside said range causing substantially no photoconductiveeffect in said face, whereby operation of said cathode ray tube altersthe refractive index of portions of said face producing a correspondingalteration in the polarization of said polarized light.

10. Apparatus as defined in claim 8 wherein said source includes a lasercavity having a plurality of modes of oscillation, and at least onereflecting surface located on the inner side of said face to cause saidmodes of oscillation to pass through said face, whereby the mode oflaser emission is selected by the operation of said cathode ray tube.

References Cited UNITED STATES PATENTS 3,164,816 1/1965 Chang 340174TERRELL W. FEARS, Primary Examiner.

US. Cl. X.R.

7. AN ELECTRO-OPTIC DEVICE COMPRISING: A CATHODE RAY TUBE HAVING A FACECOMPOSED OF A MATERIAL EXHIBITING AN ELECTRO-OPTIC AND PHOTOCONDUCTIVEEFFECT; AND CONTROL MEANS FOR ILLUMINATING SAID FACE TO ALTER THECONDUCTIVITY THEREOF.